As Rob says Borax and water will work just fine. I also like to use
borax and alcohol but it is a bit hazardous so some obvious care is
required.

I boil about 4 ounces of water and stir in powdered borax from
Walgreen’s until the water won’t take anymore into suspension. I
store it in a jelly jar with a good lid. I give it a good shake
before I use it.

For my applications I find it makes a great firescale barrier as
well as an OK flux.

Is it possible to make your own flux? And will it react the same
way as paste flux?

The usual white paste fluxes sold for jewelry (or industrial brazing
too), are rather more complex formulas than you’re likely to make
yourself. Very active stuff, but generally needs more attention to
good ventilation that the fluxes you might mix up yourself. These
generally contain fluoride containing components, though some in a
form where free fluoride gasses are not so much given off on heating
(or so they claim on the “fluoride free” labels).

Those fluxes are great for metals that oxidize very easily, the
various copper based alloys, and the like. If you’re working with
silver or gold, while those fluxes are useful for difficult joins
(generally meaning poor fits, or you’ve got existing oxides on the
metal making solder flow difficult, or the like), you usually don’t
need such aggressive fluxes. The downside to those aggressive fluxes
(the white paste fluxes in general) is that they will deplete or
burn off with more extended heating, and sometimes, while promoting
solder flow, may not actually prevent fire scale or fire stain.

As others have noted, simple boric acid or borax are both reasonably
good fluxes.

Long lasting, cheap, etc. But sometimes, if your goal is to prevent
fire stain on standard silver (not an issue with Argentium or
Sterlium + silvers), then you may find simple borax or boric acid
hard to get good coverage. (For gold, you don’t need more than simple
boric acid in alcohol, though most add some specific soldering flux
too)

For that, an easy to mix up flux is a mix of three parts boric acid
and two parts each of borax and Trisodium phosphate (sold as TSP).
TSP is sometimes hard to source these days. It used to be commonly
found in hardware stores sold as a fairly aggressive and alkaline
cleaning material commonly used to clean walls prior to painting. But
it’s a high phosphate material, and runoff can contribute to water
pollution in rivers and lakes (promotes algae growth and the like),
so currently, actual TSP is not so widely used. Instead, one finds
alternative cleaners sometimes labeled with the letters TSP as part
of the label, so read the contents carefully. If you cannot find
actual TSP, the alternative similar sounding stuff will NOT work. If
you can find Cascade dish washing powder (used in automatic dish
washing machines), the type in the familiar green box, this often is
based mostly on TSP, and if so, it works as well, or sometimes even
better, than straight TSP. But again, regulations to limit water
pollution can make this harder to find. The Cascade must be labeled
to show it is not free of phosphate, and I’m unclear just how to tell
the difference between old (that work) versions and newer phosphate
free versions required in some states.

But anyway. The mix is 3:2:2, with boric acid being the larger
component. I normally mix about 60 grams boric acid and 40 grams each
of the borax and TSP to a quart (or liter) of water. Boil the water
to fully dissolve. If not all dissolves, add more water. More dilute
is just fine.

This mix is called “Prips” flux, named for John Prip, a longtime
teacher of metalsmithing and silver smithing who first introduced the
mix to his students in the 1060s. I learned it in Fred Fenster’s
sophomore metals class at the University of Wisconsin back in '72.
Been using it since.

Prips flux is not best applied with a brush. Since the aim is
overall protection of the metal, not just soldering, you want the
whole piece of metal covered. So this is best applied with a sprayer.
Most effective are the simple small "two tubes with a hinge between)
mouth blown atomizers used by ceramicists to spray liquid glazes.
Some perfume sprayers work the same way. Also some air brushes.

The big advantages of these are no tiny orifice to clog with dried
flux, and a fine even spray. You heat the metal enough so a light
spray of the flux dries on contact to a white slightly crusty film.
Cover the metal with enough so the metallic shine is hidden. More
than that is not needed. Proceed to solder or anneal.

If this sounds like a fuss, well, it can be, but once you figure
this out and get used to it, fire scale and fire stain won’t be
problems again. Of course, you can also just switch to the more
costly fire scale and fire stain resistant silvers, like Argentium
and Sterlium plus.

Prips flux is not as active as the white paste fluxes, so a good
fit, and clean metal are even more important, but that’s standard
good soldering practice anyway. And while this may seem more fuss,
the raw materials of the flux are really cheap. I mix up Prips flux
by the quart about every year or two, and the boxes of chemical I
bought (the borax in the laundry aisle of the grocery, the TSP at
Home Depot, and the Boric acid at another hardware store sold as an
insecticide to kill roaches…) have lasted me a good ten years now.
A lot cheaper than Handy Flux or Dandix or the like…

If you search the Orchid archives for “Prips” flux (or "Pripps"
flux, a typo that seems to have sometimes propagated. Proper spelling
is one p at the end.) You will find other articles, pretty much like
this, by me or others over the years that may have more details that
I’ve left out.

However, when you mentioned…" But sometimes, if your goal is to
prevent fire stain on standard silver (not an issue with Argentium or
Sterlium + silvers), I would caution everyone that it is still a very
good idea to use Prip’s flux when working with Sterlium. I work a lot
with it and use Prip’s all the time on my sheet. Stuller, who
supplies Sterlium recommends using it as well. I do not often work
with Argentium and cannot speak authoritatively on it but, see
nothing wrong with using it then either. Just as a precaution. Cheers
from Don in SOFL at The Charles Belle Studio.

But sometimes, if your goal is to prevent fire stain on standard
silver (not an issue with Argentium or Sterlium + silvers), I would
caution everyone that it is still a very good idea to use Prip's
flux when working with Sterlium.

Thanks Don.

I worded my comment poorly on this. With Argentium and Sterlium, you
don’t get the fire stain, the result of copper oxides that penetrate
beyond the surface to some depth into the metal. For those using the
new silvers, fire stain is different from fire scale, which is the
surface black oxide that forms and is easily seen on heating. It
comes off in the pickle, leaving the distinctive lovely matte white
color that only silver can give. Fire stain, on the other hand, is a
subsurface problem, caused by oxygen being absorbed down into the
surface of the metal, and reacting with the copper that’s below the
surface. It often doesn’t show up until you’re doing your final
polish, when this annoying blotchyness of clear silver and a slightly
different color, an almost pinkish/creamy color that doesn’t polish
quite as highly, shows up. Because it penetrates the surface, it can
take quite aggressive measures to remove. Thus the desire to prevent
it’s formation with standard sterling. But just because Argentium and
Sterlium don’t form that fire stain (they don’t have the needed
copper to do it), they nevertheless benefit from being protected
from oxidation of the surface during soldering, and the solders
themselves, very much need the protection of fluxes while soldering.
Even if these alloys don’t have the copper, the silver itself can
oxidize or form black sulphides, and the germanium additives can also
be affected by oxygen. So working cleanly, and avoiding excessive and
unneeded exposure to air/oxygen while heating is beneficial, just as
it can be with even high karat gold.

But just because Argentium and Sterlium don't form that fire stain
(they don't have the needed copper to do it)

It was my understanding that Argentium still contains copper, but
the germanium oxidizes preferentially, and the oxide is transparent.
Same with surface oxidation - a layer of transparent germanium oxide
protects the silver.

I’ve been working in Sterlium for the last 2 years and grown to like
it very much. I use Dandix or Grifflux, for its what Stuller
carries. I find it works just as fine, but then my work is solder
intensive.

Now, of course, everyone will have to shake it, just to find out
what happens :-)

I can tell you. The boric acid not in solution will partially melt
and stick to your piece. Your solder join may or may not work, but
you’ll be soaking that piece in pickle until the cows come home to
get those little clear balls of boric acid off your piece (and will
end up using your thumb nail and maybe a piece of wood (I use bamboo
skewers for lots of tasks in the shop: that would be one), to
physically remove the stuck-on BA.

But just because Argentium and Sterlium don't form that fire stain
(they don't have the needed copper to do it)

It was my understanding that Argentium still contains copper, but
the germanium oxidizes preferentially, and the oxide is
transparent.

perhaps I should have said “enough of the needed copper” or some
such. Frankly, the exact details of these alloys I don’t have handy,
but the basics is that they don’t form that fire stain. In the case
of Sterlium plus, it’s alloyed with 4% copper and 3% zinc, rather
than the germanium in Argentium. I’m guessing much the same happens,
with the zinc keeping the copper from oxidizing as well.

Whatever the details, the important point is no gd dmn firestain!.
So long as that works, I’m happy.

I always grab a few extra wooden coffee stirrers and keep them around
for lots of reasons. One in particular is that I stack them up and
use them as a gauge when I rough cut prongs in a large prong setting.
I also use old playing cards as a gauge adding or removing cards to
raise or lower a bezel cup or some other object on which I want to
scribe a level line. You don’t have to get all of your tools from a
catalog. Thanks. Rob

I hadn’t seen the Sterlium composition before, but it’s interesting.
I have a ring, a pre-notched setting marked “Ster”, at least 40 years
old, without a hint of tarnish. It’s quite brittle - I broke a prong
trying to set a stone way back then, and set it aside. A jeweler I
knew suggested that it might be alloyed with zinc.